In its broader applications, the field of the present invention relates to means for verifying proper operation of electrical computing circuitry employed for pulse train manipulations or for establishing a comparative relationship between plural pulse train generating means exposed to similar inputs. The invention also relates to the measurement of variable parameters. In a particular application described herein, the present invention relates to systems and methods for measuring the volume of fluid flowing in a flowline, compensating for various factors which affect the measurement, and verifying proper functioning of the compensating means.
The prior art is replete with systems for measuring fluid flow in a flowline. Many of such systems are capable of automatically compensating for changes in various parameters of the fluid, such as pressure and temperature, which may affect the measurements. Such compensation provides a corrected and standardized output value for the measured fluid volume. Corrections are required because of errors in the flow meter operation. Standardization is required since the volume of liquids varies with changes in temperature and the volume of gases varies with both temperature and pressure. Standard U.S. volume measurements of petroleum fluids are currently based on a temperature of 60.degree. F. and atmospheric pressure at sea level. As used herein, the term fluid is intended to encompass both liquids and gases.
Certain of the prior art systems employ a flowmeter to measure flow rate through the metered flowline and transducers to measure the temperature of the fluid. The flowmeters are generally of the type which generate a series of pulses at a frequency which is representative of a measured volume of fluid. An output device counts the pulses to determine the volume. In one prior art system, the temperature transducer output is employed to increase or decrease the frequency of the pulses supplied to the output device from the flowmeter. The output device counts the number of resulting pulses to provide a temperature standardized value for volume.
One prior art system compensates for temperature effects by adding a burst of high frequency pulses to the square wave pulse train being emitted from the flow meter. The high frequency pulses are timed to occur in the period between two adjacent square wave pulses. The output device totals the high frequency pulses as well as the lower frequency square wave pulses to obtain the compensated volume. The required sensitivity of the output device to high frequency signals makes the system susceptible to noise. Efforts directed toward reducing noise distortion increase the complexity and expense of the compensating circuitry. Moreover, the requirement for inserting the high frequency bursts into the interval between adjacent square wave pulses places a practical upper limit on the square wave pulse rate. If the pulse rate is too high, not enough time between adjacent pulses is available for insertion of a relatively large number of compensating spikes.
In the petroleum industry, accurate measurement of petroleum fluids is of great economic importance which explains the need for compensation devices. The loss or addition of even a single pulse in a pulse train may affect the output reading of the metering system by a substantial amount. To ensure accurate measurement, the compensating devices must be periodically tested. If the measuring function of the system must be interrupted during testing, important economic loss may result. Testing is also an expensive requirement where sophisticated test equipment and experienced technical personnel are required to perform the tests. Conventional systems which insert a burst of high frequency pulses between square wave pulses are extremely difficult and expensive to test.